1. Field of the Invention
The present invention relates to a high voltage generator effectively useful for obtaining a high voltage by boosting a DC voltage supplied from a power source.
2. Description of the Prior Art
In a television receiver, a voltage on the order of 10 kV needs to be supplied as an anode voltage to a cathode-ray tube (CRT).
Normally such a high voltage can be obtained by boosting and rectifying flyback pulses generated in a horizontal output circuit. However, some of the recent television receivers are so contrived that high voltage pulses from a separate high voltage generator based on the same circuit principle as that of a horizontal output circuit are boosted and rectified to produce a high voltage of 30 kV or so which is applied as an anode voltage.
FIG. 6 shows a conventional example of such high voltage generator, wherein there are included a choke coil 1 to which a voltage +B is supplied, a switching transistor 2, a rectifier diode 3, a smoothing capacitor 4, a flyback transformer 5, a pulse generating transistor (hereinafter referred to as output transistor) 6, a damper diode 7, and a resonance capacitor 8.
Flyback pulses generated in a primary coil L1 of the flyback transformer 5 are boosted by a secondary coil L2 thereof and, after conversion into a DC voltage by a high voltage rectifier circuit 9, it is applied as an anode voltage to a cathode-ray tube CRT.
Denoted by 10 is a control circuit for first detecting the anode voltage, then pulse-width modulating the detected voltage and controlling the intermittent on-off action of the switching transistor 2.
In the above conventional example, the DC output voltage of a pulse-width controlled power supply regulator constituted of the aforementioned choke coil 1, switching transistor 2, rectifier diode 3 and smoothing capacitor 4 is supplied to a high voltage generator constituted of the aforementioned flyback transformer 5, switching transistor 6, damper diode 7 and resonance capacitor 8. And when the high voltage output level is varied to rise, the pulse width of the transistor 2 is so controlled as to lower the DC output of the pulse-width controlled power supply regulator, whereby the high voltage output level is maintained constant under control.
However, in the known high voltage generator mentioned above, there exists a problem that, since the DC output voltage is controlled by the pulse-width controlled power supply regulator to control the level of the flyback pulses generated from the flyback transformer 5, it becomes necessary to provide an expensive choke coil 1 and a large-capacity smoothing capacitor 4 to consequently bring about an increase in the production cost.
Furthermore, when there occurs any variation in the high voltage output level, it is difficult to rapidly change the DC output of the pulse-width controlled power supply regulator in compliance with such output level variation, hence failing in improvement of the response characteristic in the high voltage regulation.
Therefore, if a white portion W of a high luminance level is existent in a television image A as illustrated in FIG. 7, a great beam current comes to flow in the CRT relative to such white portion W, and after a resultant drop of the high voltage, the normal high voltage cannot be restored fast in a short time. Thus, the anode voltage is lowered to consequently expand the contour line of the white portion W as represented by a dotted line, hence raising a problem of display distortion caused in the image.